A feedback loop in a conventional regulator system typically uses voltage feedback and a resistive voltage divider to set the regulated output voltage relative to an input reference voltage. The difference of these two signals (i.e., the regulated output voltage and the reference voltage) is usually obtained by standard connections in an operational amplifier (“op amp”), differential amplifier, or transconductance amplifier, which operate on the voltage signals.
FIG. 1 shows a conventional op amp- or differential amp-based voltage regulator 10. A voltage divider 30 (comprising first and second resistors 32 and 34 in series between a regulated voltage VOUT and a ground potential 36) provides a first input into the op amp/differential amp 20. A conventional bias source 40 (e.g., a conventional bias voltage generator) provides a second input (i.e., a reference voltage VREF) into the op amp/differential amp 20. The difference ΔV between the two input signals is output to the signal path having a node at which the voltage (VOUT) is regulated, thereby providing a feedback path to the voltage-controlled voltage source 10.
In the example shown in FIG. 1, the ground potential 36 in the voltage divider 30 is a system potential, whereas the ground potential 42 for the voltage source 40 is a reference ground. The different ground potentials may have different values due to different noise effects (e.g., from the system vs. on the chip). As a result, when the feedback loop is closed, the regulated voltage VOUT has a value that can be defined according to the following Equation (1):VOUT=(VREF±ΔGND)(1+(R2/R1))  (1)where ΔGND is the voltage difference between the different ground potentials 36 and 42, R1 is the resistance of resistor 32, and R2 is the resistance of resistor 34.
In a relatively high-gain, high-power system, R2/R1>>1, andVOUT=(VREF·(R2/R1))±(ΔGND·(R2/R1))  (2)
In such a system, the sensitivity of the regulated voltage VOUT to ground noise is:dVOUT/dΔGND=R2/R1  (3)
In many systems, it is difficult to maintain a solid ground reference between the output voltage and reference voltage. For example, in a white LED (WLED) backlighting system, the DC ground reference for the output voltage in a boost regulator IC is external to the IC, whereas the voltage reference signal is internal. This creates noise susceptibility and, in a high power system, erratic regulator behavior, particularly if the ratio of the output voltage to the reference voltage is large. In many boost converter applications, the output voltage to reference voltage ratio can be as high as 40:1. This means a ground noise level of 100 mV shows up on the regulated output multiplied by 40× (i.e., 4V).
FIG. 2 shows a voltage-controlled transconductance control circuit 10′. When the input VOUT is part of a feedback loop from a node in the signal path being controlled, the control circuit 10′ and the feedback loop together may be considered to be a regulator. The transconductance control circuit 10′ includes a transconductance amplifier 20′, and operates similarly to the op amp-based regulator 10 of FIG. 1, except that the output current ΔI from the transconductance amplifier 20′ controls or biases a current source 50, which outputs a current IOUT having a value equal to the gain of the transconductance amplifier 20′ times the voltage VFB from the voltage divider 30. However, the value of voltage VOUT is still defined according to Equation (1) above. As a result, variations in the different ground potentials can cause significant variations in the regulated current output from the transconductance control circuit 10′.